1. Field of the Invention
The present invention relates generally to an apparatus for mixing a first fluid into a second fluid, and particularly to an apparatus for mixing a fluid, preferably a gaseous fluid such as, for example, steam, ozone or oxygen gas, into a cellulose pulp suspension.
2. Description of the Prior Art
The heating of liquids and suspensions by means of direct steam can be difficult to carry out for a large number of reasons. One is the difficulty of atomizing the steam and simultaneously keeping the suspension in such motion that a smooth and continuous condensation takes place, which requires, namely, that the steam is evenly atomized in the liquid or suspension. This is especially difficult when a large quantity of steam is supplied. When steam is added, it occurs, moreover, that the volume of the steam bubbles can be so great that the convection of heat between the steam and the liquid is insufficient for the desired continuous condensation. Because of this, intermittent, violent steam implosions arise, causing shocks and vibrations. These can be so violent that mechanical damage is incurred. The damage is accentuated as the quantity of steam to be added increases.
In general, a number of requirements can be placed upon a steam mixer. The steam should be added such that local surpluses do not occur during passage through the mixer. The degradation or so-called fluidization must take place in such a way that local pressure variations are minimized. Any implosions which occur because of steam bubbles should take place in a section in which the components or the construction material cannot suffer damage resulting from the cavitation-like phenomena. The mixer should have some form of in-built elasticity to enable it to absorb pressure and shocks caused by possible momentary disturbances in the steam and pulp flows up to and through the mixer.
A large number of apparatuses for mixing a gaseous fluid into a pulp suspension are known. The Swedish Patent No. 468 341 describes an apparatus for mixing a suspension of a cellulose-containing fibre material and a fluid such as, for example, gases in the form of ozone, oxygen and chlorine and liquids containing various active substances, e.g., chlorine dioxide. In its basic principle, this mixer comprises a funnel-shaped part and, within the funnel-shaped part, a conical moving part. Between the funnel-shaped part and the conical part there is formed an adjustable gap through which the pulp passes. In the walls of the funnel-shaped part there are located a number of openings for the fluid which is to be mixed into the passing pulp. Drawbacks with this apparatus are that it is relatively large, that its installation, especially in existing pipe systems, is complicated, since the flow direction of the pulp alters in the mixer, requiring that the pipe system to which the mixer is to be connected has to be redirected, and that the mixer requires some form of stand or base on which to be mounted.
Also common are mixer devices having a rotary part for mixing fluid into the pulp. A problem in these devices is that the rotation gives rise to large pressure variations, which create local zones of very low pressure to which the steam makes its way, resulting in implosions as described above. A further problem is to distribute the steam evenly in the pulp suspension, especially when large quantities of steam are to be supplied, as a result of which capacity problems can also arise.